Two metal oxides, examined on their optics, their chemistry and their limits
What zinc oxide and titanium dioxide actually do, why the absorb against reflect story is a simplification, what particle size changes, what the nano labelling rule requires, and why white cast is physics rather than a formulation failure.
What zinc oxide actually does in a sunscreen
Zinc oxide is a semiconductor that absorbs ultraviolet across its band gap. What that means for coverage, for texture and for the rest of the formula.
What titanium dioxide actually does in a sunscreen
Titanium dioxide is the strongest white pigment in industrial use and a competent UV filter. The two facts are connected, and they explain both its strengths and its problems.
Zinc oxide and titanium dioxide, compared on what actually differs
The two permitted mineral filters are not interchangeable. They differ in spectrum, in whiteness, in formulation behaviour and in the questions still open about them.
Absorb against reflect: the simplification taught to every shopper
Mineral filters reflect, chemical filters absorb. It is the tidiest explanation in skincare and it does not survive measurement of what mineral films actually do.
Particle size: the variable that decides how a mineral sunscreen looks and works
Nothing else in a mineral formulation changes as much with one number. Particle size governs whiteness, ultraviolet performance, dispersion stability and the whole nano question.
Nano and non nano: what the labelling rule actually requires
A nanomaterial in a cosmetic must be identified in the ingredient list. What that marker means, what it does not mean, and why non nano is a marketing term rather than a legal one.
The nanoparticle safety questions, separated into the settled and the open
Dermal application of nanoscale metal oxides to intact skin has been assessed repeatedly. Inhalation, damaged skin and environmental fate are where the live questions sit.
The white cast problem, and why the physics makes it hard to solve
White cast is not a formulation failure. It is the same optical property that makes these materials useful, expressed at visible wavelengths, and it can be reduced but not eliminated.
Why mineral formulations are so much harder to make elegant
Suspending a high loading of dense inorganic particles in a stable emulsion that spreads well is one of the harder problems in cosmetic formulation. Here is what makes it hard.
Mineral sunscreen and darker skin tones: a formulation problem, not a preference
A pale scattering veil is far more visible against deeper skin. That is optics, and the usual remedy creates a shade range problem that the category has handled badly.
The quantity problem: how a cosmetic complaint becomes a protection gap
Sun protection factor is measured at an application density heavier than most people use. In the mineral category, the reasons to apply thinly are stronger.
Which page answers what
This section is an index, not an argument. The eleven articles above examine zinc oxide and titanium dioxide on their optics, their chemistry and their limits, and each question below is answered in one place rather than in several. Three of the answers sit outside this section, because they are questions about who a mineral formulation suits and what a pack may say rather than about the filters themselves, and they are linked here so the chain is not broken.
| The question | The page that answers it |
|---|---|
| Is zinc oxide better than titanium dioxide, or are they interchangeable? | Zinc oxide and titanium dioxide, compared on what actually differs |
| Do mineral filters really sit on top and reflect ultraviolet? | Absorb against reflect |
| What does the nano or non nano marker on the ingredient list mean? | Nano and non nano: what the labelling rule actually requires |
| Which nanoparticle safety questions are settled and which are still open? | The nanoparticle safety questions, separated |
| Why does it leave a white film, and can that be formulated out? | The white cast problem |
| Why does it read grey or ashy on deeper skin? | Mineral sunscreen and darker skin tones |
| How much do I have to apply for the stated factor to hold? | The quantity problem |
| Is mineral the right choice for a baby or a child? | Mineral sunscreen for babies and children, in the skin section |
| Is mineral gentler on sensitive skin? | Is mineral better for sensitive skin, in the skin section |
| What are the words on a UK mineral pack allowed to mean? | The UK label reference |
| Does this pack protect against UVA as well as UVB? | What broad spectrum has to satisfy on a UK pack |
What this section does not do: it names no brand and judges no product, here or anywhere on this publication. It does not rank the two oxides against each other as a verdict, because the comparison that matters is spectrum, whiteness and formulation behaviour, which is what the comparison article sets out. Questions about the environment sit in the environment section.
One question sits outside this section and decides how a mineral pack should be read: whether it carries the UVA mark. What broad spectrum has to satisfy on a UK pack sets out the one third ratio and the critical wavelength criterion behind that mark, and why a mineral formulation is not exempt from either.
Editorial disclosure. This article contains no commercial links. Nothing on it has been paid for, no company has been given sight of it, and no brand or product is named anywhere on this site. This publication takes no money from sunscreen or skincare companies at any price, and the reasons are set out in what we refuse to sell. Published independently.
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